Microbiology, 2021

1108 26 • Nervous System Infections
L. monocytogenes is generally introduced into food items by contamination with soil or animal manure used
as fertilizer. Foods commonly associated with listeriosis include fresh fruits and vegetables, frozen vegetables,
processed meats, soft cheeses, and raw milk. 15 Unlike most other foodborne pathogens, Listeria is able to grow
at temperatures between 0 °C and 50 °C, and can therefore continue to grow, even in refrigerated foods.
Ingestion of contaminated food leads initially to infection of the gastrointestinal tract. However, L.
monocytogenes produces several unique virulence factors that allow it to cross the intestinal barrier and
spread to other body systems. Surface proteins called internalins (InlA and InlB) help L. monocytogenes
invade nonphagocytic cells and tissues, penetrating the intestinal wall and becoming disseminating through
the circulatory and lymphatic systems. Internalins also enable L. monocytogenes to breach other important
barriers, including the blood-brain barrier and the placenta. Within tissues, L. monocytogenes uses other
proteins called listeriolysin O and ActA to facilitate intercellular movement, allowing the infection to spread
from cell to cell (Figure 26.10).
L. monocytogenes is usually identified by cultivation of samples from a normally sterile site (e.g., blood or
CSF). Recovery of viable organisms can be enhanced using cold enrichment by incubating samples in a broth
at 4 °C for a week or more. Distinguishing types and subtypes of L. monocytogenes—an important step for
diagnosis and epidemiology—is typically done using pulsed-field gel electrophoresis. Identification can also be
achieved using chemiluminescence DNA probe assays and MALDI-TOF.
Treatment for listeriosis involves antibiotic therapy, most commonly with ampicillin and gentamicin. There is
no vaccine available.
Figure 26.10 (a) An electron micrograph of Listeria monocytogenes infecting a host cell. (b) Listeria is able to use host cell components to
cause infection. For example, phagocytosis allows it to enter host cells, and the host’s cytoskeleton provides the materials to help the
pathogen move to other cells. (credit a: modification of work by the Centers for Disease Control and Prevention; credit b: modification of
work by Keith Ireton)
CHECK YOUR UNDERSTANDING
• How does Listeria enter the nervous system?
14 Scallan, Elaine, Robert M. Hoekstra, Frederick J. Angulo, Robert V. Tauxe, Marc-Alain Widdowson, Sharon L. Roy, Jeffery L. Jones,
and Patricia M. Griffin, “Foodborne Illness Acquired in the United States—Major Pathogens,” Emerging Infectious Diseases 17, no. 1
(2011): 7-15.
15 US Centers for Disease Control and Prevention, “Listeria Outbreaks,” 2016. Accessed June 29, 2016. https://www.cdc.gov/listeria/
outbreaks/index.html.
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